The 1:1 ion pair formation constants (K IP 0 ) of 1-alkyl-3-methylimidazolium ([RMeIm] + ; R ) butyl, hexyl, and octyl) and 1-butyl-2,3-dimethylimidazolium ([BuMe 2 Im] + ) ions with tetrafluoroborate ([BF 4 ] -), hexafluorophosphate ([PF 6 ] -), bis(trifluoromethanesulfonyl)amide ([NTf 2 ] -), and 2,4,6-trinitrophenolate (picrate, [Pic] -) ions have been determined conductometrically in dichloromethane at 25 °C. The K IP 0 determinations have also been made for symmetric tetraalkylammonium ions ([R 4 N] + ; R ) methyl, ethyl, propyl, and butyl) for comparison. For a given anion, the K IP 0 value of the [RMeIm] + salt is almost independent of the length of the alkyl chain (R), whereas that of the [R 4 N] + salt decreases with increasing alkyl chain length. Such a difference in the alkyl chain length dependence of the ion pair formation ability can be explained on the basis of the structures of the ion pairs calculated by density functional theory. The K IP 0 values of [BuMeIm] + , [BuMe 2 Im] + , and [Et 4 N] + , which are similar in the van der Waals volume, are in the order of [BuMeIm] + . [BuMe 2 Im] + ≈ [Et 4 N] + , showing that the C2-H atom on the imidazolium ring makes an important contribution to the strong ion pair formation ability of [RMeIm] + . For a given cation, the K IP 0 value is generally smaller for the larger anion, i.e., [BF 4 ]g [PF 6 ]g [NTf 2 ] -> [Pic]for [Et 4 N] + and [BuMe 2 Im] + , and [BF 4 ] -> [PF 6 ]g [Pic]g [NTf 2 ]for [RMeIm] + .
We have performed the heat capacity, neutron diffraction, and neutron quasielastic scattering measurements of an ionic liquid 1-octyl-3-methylimidazolium chloride (C8mimCl). The heat capacity data revealed that C8mimCl exhibits a glass transition with a large heat capacity jump at T(g) = 214 K, which is lower than T(g) of C4mimCl with a shorter alkyl-chain. In the neutron diffraction measurement for a deuterated analogue, d-C8mimCl, the peaks associated with the inter-domain, inter-ionic, and inter-alkyl-chain correlations appeared at (3, 11, and 14) nm(-1), respectively. The temperature dependence of these peaks indicates that the packing of the alkyl-chains becomes more compact and the domains become more vivid and larger as decreasing temperature. The quasielastic neutron scattering measurements using neutron spin echo and time-of-flight type instruments demonstrated that C8mimCl has faster relaxations probably owing to the alkyl-group and a slower relaxation owing to the ions. The latter relaxation, which is related to the glass transition, is of non-exponential as in the α relaxation of glass-forming molecular liquids. The relaxation of domains could not be observed in the present experiment but should have relaxation times longer than 100 ns. This is the first report to clarify temperature dependence of the hierarchical structure and relaxations simultaneously for a typical ionic liquid.
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